A stereoscopic 3D display usually presents an observer with images with parallax from individual right and left eye viewpoints. There are two methods of providing the two eyes of the observer with the parallax images in a time sequential manner. In one method, the observer utilizes a pair of shutter or 3D glasses which transmit or block light from the viewer's eyes in synchronization with alternating the left/right image display. Similarly, in another method, right eye and left eye viewpoints are alternatively displayed and presented to the respective eyes of the observer but without the use of 3D glasses. This second method is referred to as autostereoscopic and is sometimes desirable for stereo 3D viewing because separate glasses are not needed though there is limited permissible head motion.
A liquid crystal display (LCD) is a sample and hold display device such that the image at any point or pixel of the display is stable until that pixel is updated at the next image refresh time, typically 1/60 of a second or faster. In such a sample and hold system, displaying different images, specifically displaying alternating left and right images for an autostereoscopic display, requires careful timing sequencing of the light sources so that, for example, the left eye image light source is not on during the display of data for the right eye and vice versa.
In order to have good left/right image separation, light is delivered alternately to each eye. An attempt to achieve this is to provide a light guide that extracts light by means of translationally invariant and symmetric prism features on a back surface of the light guide. Since the extraction features are the same over the extent of the light guide, the amount of light extracted when either the left or right side light source is illuminated decreases, more or less exponentially, as the light guide is traversed. This construction can often provide a non-uniform illumination of the stereoscopic 3D display and resulting in a degraded 3D viewing experience. The viewer perceived 3D image will be uniform across the display as long as the individual left side and right side non-uniformities do not exceed 5 to 1, or more preferably 3 to 1.